My web designer tells me that in a web page, the empty margins or padding should always be multiples of a standard. For example 6 px, 12px, 18px. This should produce nicely balanced lay-outs. I would like to learn a little bit more about it:

The reason for the bounty from me is I think this question is very interesting, and needs to be answered correctly. My answer is just one from my point of view. I think that there is more appropriate ones, taking full act on the complexity (like @Coswyn).
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Benny Skogberg♦Apr 25 '11 at 9:16

"empty margins or padding should always be multiples of a standard" (emphasis mine) - usage of the word "always" is a pretty good indicator of a false statement, because absolute statements are always false.
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zzzzBovJul 25 '12 at 17:38

8 Answers
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The best learning resource for this would be a good introduction on typography – probably the seminal classic by Bringhurst (see http://webtypography.net for a good roundup applied to the web), though e. g. Spiekermann's ‘Stop Stealing Sheep…’ is not bad for starters, either – and on design grids (see my answer here on UXexchange).

When designing grids you are mostly using a basic module (proportions ideally defined by working from the content outwards) that all content is fitted to (i.e. multiples of it).

Vertical and horizontal margins between blocks are in most cases different. Vertical whitespace is often oriented on the baseline grid (see e.g. Bringhurst, again). Using one is highly recommended to achieve at a unifying vertical rhythm.

The minimum amount of horizontal whitespace, i. e. primarily the separation between columns of body text (gutter), is governed by Gestalt psychology with font size, line spacing, and line width as main influencing factors. You should place text blocks far enough from another such that your recipients will be able to see them as distinct units of their own. A traditional rule of thumb would make the gutter at least 1.5 ems wide in order to appear significantly wider than any possible whitespace within a line of text. On the screen, good line spacing tends to be a little bit wider than in traditional print, though. Hence you will probably need a little bit more than that. Using the same value as your baseline grid is a good guess to start with in most cases.

BTW: design standards – unless significantly backed up by ergonomics or cognitive psychology – are never standards in the more rigid sense of the word. You may violate any ‘standard’ as long as you know why you are doing it.

It can be helpful to have standard spacing within content sections. IE paragraphs, headings and lists should all have the same amount of spacing below them as that creates rhythm. The rule of thumb though is to maintain balance and consistency.

This site's layout is a good example of how a combination of adherence to multiple patterns yields balanced proportions and an overall consistent structure. The logo, for example, is spaced approximately 14px at the top and bottom within its header segment though it's indented 18px from the edge of the question header, making the top and bottom spacing approximately 30% smaller than the left side - other elements within the header are spaced similarly. On the other hand, the spacing of the text within the question header is approximately 9px on the bottom, 13px on the top, and 24px on the left side. That sets the top spacing to be approximately 50% smaller than the left side, and the discrepancy in the bottom is relative to the alignment of the text to the bottom of the main header in addition to it being header text (separated from the main content with a divider, though closer to that content which it more closely relates to than anything else that surrounds it).

Proportions vary and are an art in themselves. There is a math to it all, but bringing a complex structure together is, in practice, a generally intuitive process.

I'm sure there are multiple books and online resources that touch on this, and they are all likely helpful in their own ways, but my best advice is to be an avid observer of proportions in the things around you - in themselves and relative to their environments and other things in that environment - especially in nature. In time you will hone your intuition and begin to more naturally imitate those proportions and apply them in their appropriate settings.

To be clear: 1em = font-size, 2em = 2 * font-size etc. Therefore: if h1 and p both have margin of 0.5em, and h1 has 20px and p 12px font-size, then heading's margin is translated to 10px and the paragraph's to 6px.
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koiyuApr 24 '11 at 11:37

But layout proportions do not scale in an optically linear manner as the perceived visual ‘blackness’ of the content varies when scaled (at least where type is concerned). Therefore, using relative units does not help that much in that respect, IMHO.
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Sascha BrossmannApr 25 '11 at 22:45

You should IMHO not mistake the technical implementation of the grid (i.e. the main reason for using a CSS framework) with the underlying design. The default values in 960gs, Blueprint etc. are exactly that: defaults, i.e. tolerable guesses to start from. But they do not release you from making a design decision that might be better suited to your content. All those frameworks are quite well adaptable to different grid setups (see the countless grid generators for them on the web).
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Sascha BrossmannApr 26 '11 at 7:28

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I think we differ here on the design side. Giving endless options to make each page as good as it could be in isolation of the others is not something that I believe makes a good UX. Consistency is far more valuable for me. I will however agree that one shouldn't stick exactly to a chosen grid even at the detriment of the page.
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JohnGB♦Apr 26 '11 at 9:59

There is value in consistency of a grid framework. The example of 960 is very valuable - people are used to that width and designers are used to working with it. Consistency is part of usability.
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Chris KluisApr 26 '11 at 10:39

ahem… where did I state that you should design ‘pages’ in isolation of each other? I had hoped that it was clear that my statement applied to designing a system :-) But the design of a good grid system always should be guided by the content it shall be supporting.
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Sascha BrossmannMay 9 '11 at 23:01

I do not believe there are any 'rules' to design, only principals; congruence and proportion are key principals of design, wherein any principal can be disregarded.

Your margins can be whatever you decide is aesthetically best, but should you choose to be mindful of "best practices," you will most likely make that decision based in an adherence to effective use of proportions, consistency and purpose.

Art is not formulaic--if every website had margins and banner sizes perfectly matched to 'strict rules,' and 'the best grid,' the originality (and overall impact) of that web design would suffer greatly.

Using 6px or 12px margins throughout your site is no different than 7px and 29px margins--just have a reason for your choice, and stick to it rationally. The people who say your margins need to be "perfect golden ratios of this containers width" are, in my opinion, losing sight of the creative aspect of web design--they also tend to be the designers who make cookie cutter websites... albeit potentially nice looking cookie cutter websites :)

Whether to use em over px is another story--and will require your own discretion. Fluid websites aren't always the best solution; it wouldn't be a bad habit to get used to using em's though (and maybe leaving yourself a reminder)--eg.

It is a common misunderstanding that px was not a relative measure. But according to the W3C it is and meanwhile even MSIE behaves accordingly. Consequently, the former main argument for preferring ems over px has become obsolete and a mere matter of personal taste as both get scaled properly. You might even prefer px now due to browser rounding errors when dealing with the float values necessary for working with ems.
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Sascha BrossmannApr 26 '11 at 7:09

Oh, and while I basically highly agree with your ‘no rules’ notion, there are some very basic ones governing visual perception, nonetheless. Hint: cognitive psychology :-) Not that these could not be creatively employed…
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Sascha BrossmannApr 26 '11 at 7:14

@Sascha - I was under the impression that em responded to user defined stylesheet font sizes, so that it scaled with respects to their input... not just the zoom feature of browsers--that's been scaling properly with px since I can remember.
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jlmakesApr 26 '11 at 9:13

@Sascha - If you mean along the lines of the Gestalt Principal, then yes--there are explicitly defined behaviors of visual perception... but doesn't cognitive psychology explore visual perception in a reactionary way? I have yet to learn of a definitive rule to make something "look good," other than maybe the use of patterns and a grid over arbitrary shapes and placement; I may be a hopeless romantic for art, lol.
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jlmakesApr 26 '11 at 9:17